The present invention generally relates to ignition systems for small gasoline engines. More particularly, the invention relates to an apparatus for measuring the conductivity of a combusted fuel-air mixture in such an engine.
Large internal combustion engines, such as automobile engines, often include various types of operational sensors. A variety of sensors can be used as inputs to a microprocessor in order to determine efficiency of combustion. In this manner, the engine may finely tune the fuel-air mixture and ignition timing presented to the engine in order to enhance engine efficiency and reduce pollutant output.
Two obstacles not encountered in large engines have impeded the employment of such measuring technology in small gasoline engines typically used on hand-held equipment (e.g., blowers and string trimmers), lawn mowers, pumps, generators and the like. First, any measuring technology employed must be inexpensive because the commoditization of the small engine places a premium on cost of production. Second, in the small engine arena, measuring technology must often operate on limited power because small engines typically do not include a battery to act as a power source. As a result, sophisticated automotive sensors requiring higher levels of power consumption are not feasible.
A currently available measuring technology—ion sensing—provides both low cost and low power consumption. In such an arrangement, the spark plug itself is generally used as the sensor. The various electronics required to detect and interpret the sensor output are simply integrated into the engine control unit or ignition system. Ion-sensing is often used for knock and misfire detection.
A typical ion sensor operates as follows. A power source supplies current across terminals located in the area where conductivity is to be measured. In the case of an internal combustion engine, the sensor is typically located in the chamber in which combustion of the fuel-air mixture occurs. A voltage that rises and falls with the conductivity of the combusted fuel-air mixture is measured across the sensor terminals.
Certain properties of ion sensors, however, have heretofore inhibited their use in carbureted engines. Specifically, unpredictable variations in both fuel quality (due to additives and oxygen content) and the variation of air-to-fuel ratio in non-closed loop air-to-fuel ratio controlled engines causes the conductivity measured by the ion sensor to vary dramatically—up to a magnitude of 100 times. Knock information, however, may be found as a high frequency oscillation overlaid on the falling edge of the ion signal.
Measuring this information with accuracy is extremely difficult. For example, components with an operating span of 0-5V are normally used in logic control circuits. For a signal-to-noise (SN) ratio of 10 to 1, a signal level of 5V means that the circuit design must assume a noise level of 0.5V. If the signal level has a dynamic span of 100 times and a SN ratio of 10 to 1 is used, then the noise level can exceed the signal level. As a result, the system would be unable to obtain information. Similarly, a signal having an amplitude greater than 5V would not yield useful information. In order to compensate for these deficiencies, much higher SN ratios and/or more powerful microprocessors would be necessary. Both of these add cost to the system.